Mini marble isn't a true species. If you know the size of the parents it may give you a good idea as to their adult size, but it's not certain. Or if they have had multiple batches of offspring that some have reached adulthood you could go off that. Like dwarfism in humans two people with dwarfism can have a regular sized offspring. It's a genetic abnormality and as such does not guarantee the final adult size of offspring. I'd prepare for a normal adult size marble and that way if it is truly a dwarf then you will be good.